The use of hollow, air cooled blades and vanes is common in modern gas turbine engines. During engine operation, air is flowed into an internal cavity of such components and is discharged through cooling holes present in the airfoil section and sometimes present in the platform and tip. The passage of air through the blade or vane (hereinafter referred to collectively as "blades") extracts heat from the blade surface, allowing use of the component even when the gas stream temperatures exceeds the melting temperature of the alloy from which the blade is made. Obstruction of the cooling holes can result in localized hot spots on the blade, which can cause cracking or localized melting of the blade; it can also cause degradation of a coating which may be present on the blade surface.
Coatings are applied to blades to provide enhanced resistance to oxidation, corrosion, erosion, and other such types of environmental degradation. See, e.g., U.S. Pat. Nos. 4,248,940 to Goward et al and 4,585,481 to Gupta et al, both of which are incorporated by reference.
In the plasma spray coating process, powders are injected into a high temperature, high velocity stream of ionized gases. At the point where the powders are injected into the gas stream, the temperature can be about 15,000.degree. F. As a result, the powders are typically molten when they strike the substrate surface.
It has been observed that when coatings are plasma sprayed onto air cooled blades, the cooling holes can become filled with coating material, requiring a subsequent machining operation to reopen the holes. This is not only time consuming and therefore expensive, but locating the exact position of each hole is difficult.
One method which attempts to prevent the closure of cooling holes during a plasma spray operation is described in U.S. Pat. No. 4,402,992 to Liebert. In this patent, a high pressure stream of gas is flowed into the internal cavity of the blade during the coating operation; the gas is discharged through the cooling holes and is said to deflect the incoming coating particles away from the holes, thereby keeping the holes open. However, unless the gas is discharged evenly through each of the holes, some holes are still likely to become plugged. Furthermore, as the gas is discharged through each hole, it may cause turbulence adjacent to the surface of the blade, which could interfere with the application of a coating having a uniform thickness. Accordingly, engineers have sought improved methods for keeping the cooling holes open during the high temperature plasma spray coating process.